1. Field of the Invention
The invention relates generally to a dental laser assembly and method for treating tooth decay. More specifically, it relates to an apparatus and method for selectively removing carious lesions and/or dental decay from human teeth without affecting the adjacent healthy dentin and enamel.
2. Discussion of Prior Art
An all too familiar problem with human teeth is tooth decay. It is well known that a common method of treating tooth decay involves anesthetizing the local area, using a mechanical drill (or similar equipment) to remove the decayed portion of the tooth, placing a restorative material in the cavity area and curing the restorative material. This traditional treatment method has many well known disadvantages. In order to overcome some of these drawbacks, it has been proposed to use a laser to assist in some of the steps of the traditional treatment method.
Over the last 25 years there have been numerous papers, patents, and studies relating to the use of various types of lasers as a medical tool. In particular, CO.sub.2, Argon and Nd:YAG lasers have been proposed for a variety of applications and have been used for a number of "soft tissue" dental procedures, for example, cutting tissue. However, "hard tissue" dental applications for the laser have been slower in coming due to a number of complex issues. "Hard tissue" dental applications include, for example, using a laser to remove enamel and/or tooth decay.
One problem heretofore encountered with "hard tissue" dental applications is the management and dissipation of excess laser generated thermal energy before such energy damages the healthy dentin, enamel, pulp, and/or tissues which are in close proximity to the decayed or carious areas to be treated. Previous studies have suggested that the use of the laser to remove tooth decay is severely limited since the power level necessary for the laser to effectively ablate or eradicate the decay produces significant amounts of heat which can kill the tooth nerve and cause other undesirable affects.
Nevertheless, a number of techniques and methods have been suggested which claim to overcome some of the drawbacks described above. For example, U.S. Pat. No. 5,055,048 issued to Vassiliadis et al. discloses an Nd:YAG laser dental assembly for use in eradicating carious lesions in teeth and for a variety of other applications. In Vassiliadis et al., the laser is excited so that the laser emits a laser beam along a predetermined axis that is in line with a fiber optic delivery system and at a pulse rate of 1 to 10,000 pulses per second and with an average power variable from 1/10th to 50 watts. A handpiece is dimensioned to be inserted into a human mouth while an optical fiber optically connects the laser output to the handpiece. The laser assembly also includes a continuous output aiming laser which, upon activation, provides a continuous laser aiming beam coaxial with the treatment beam. Both beams are introduced into the fiber optic delivery system.
In the BACKGROUND OF THE INVENTION section of the Vassiliadis et al. patent, the inventors discuss the use of Nd:YAG laser systems for medical surgical applications. It is disclosed that these previously known laser systems typically have been constructed to provide very high average powers, i.e., in the range of 60 to 100 watts of continuous power. However, various drawbacks of this type of system are asserted. It is further asserted that there has never been an Nd:YAG laser system particularly suited for dental laser applications such as eradication of carious lesions and other applications.
As disclosed in U.S. Pat. No. 5,002,051 issued to Dew et al., optical energy generated by lasers has been applied in recent times to various medical and surgical procedures because the monochromatic and coherent nature of the light generated by lasers has been shown to have absorbency characteristics which vary with the nature of the illuminated tissue. Thus, for a given tissue type, the laser light may propagate through the tissue, substantially unattenuated, or may be almost entirely absorbed. The extent to which the tissue is heated, and ultimately destroyed, depends on the extent to which it absorbs the optical energy. According to Dew et al., it is generally preferred that the laser light be essentially transmissive in tissues which are desired not to be affected, and absorbed by the tissues which are to be affected.
It is further stated in Dew et al. that a known advantage of a laser system is that the optical energy can be delivered to the tissues desired to be operated upon in a precise location and at predeterminable energy levels. The precision with which the laser energy can be directed is enhanced by its ability to be guided by known thin optical fibers. Various types of lasers are discussed. For example, it is stated that argon lasers emit energy at 0.488 and 0.515 micrometers, carbon dioxide (CO.sub.2) gas lasers emit energy at a wavelength of 10.6 micrometers and that Nd:YAG lasers have a predominate mode of operation at a wavelength of 1.06 micrometers.
Other patents also disclose using a laser for various dental applications. For example, U.S. Pat. No. 4,818,230 issued to Myers et al. discloses a method for removing dental decay and carious lesions from human teeth comprising the steps of aiming a pulsed laser so that its output impinges upon the decay and repeatedly activating the laser in a pulsed mode until the decay is eradicated from the tooth. The use of a YAG laser with an energy output of 0.1-100 millijoules is disclosed.
U.S. Pat. No. 4,521,194 issued to Myers et al. also discloses a method of removing carious lesions and/or stain from human teeth comprising the steps of aiming a YAG laser so that its output impinges upon the lesion and/or stain and thereafter repeatedly activating the laser in a pulsed mode until the undesired characteristic is removed. The YAG laser is disclosed as producing an energy output of 1-100 millijoules.
U.S. Pat. No. 5,020,995 issued to Levy discloses the use of a laser for removal of tooth and gum tissue. Levy discloses that the enamel and dentin of a tooth include, as one component, hydroxyapatite, which is in amorphous form in the dentin and crystalline form in the enamel. Levy further discloses that healthy dentin is in mineralized form, while dentin which has experienced decay is in demineralized form. Levy also discloses that it has been found that hydroxyapatite absorbs laser radiation in the wavelength range of 9-11 .mu.m, such as produced by CO.sub.2 lasers, and also in the wavelength range 0.5-1.06 .mu.m, which includes the wavelength that can be produced by a YAG laser.
Levy further states that laser radiation absorption by the various parts of a tooth at various wavelengths is influenced by the absorption of the radiation energy by the water component thereof and that the greater the absorption by water, the less energy is available for absorption by the other components. Levy also states that is has been found that radiation at a wavelength of 1.06 .mu.m (the output of a Nd:YAG laser) is absorbed to a lesser degree by water, and therefore has a greater effect on mineralized tissues. It is said that laser radiation at a wavelength of 0.532 .mu.m (the output of a frequency doubled Nd:YAG laser) is not absorbed at all by water and can be effective on mineralized tissues if a sufficiently high, and thus dangerous, power level is employed. Levy further states that while a particular wavelength may inherently have a cutting effect on enamel or dentin, it has been found that the practical utilization of radiation at such a wavelength for dental procedures is highly dependent on the form in which the radiation is applied, with respect to the energy level, pulse duration and repetition rate. Specifically, it is stated that efforts to apply such radiation in the form of high energy pulses of short duration have been found to produce a highly localized temperature increase, resulting in differential thermal expansion which can cause mechanical damage to the tooth as well as vascular damage to pulp tissue. Conversely, low energy pulses of long duration are said to cause a more widespread heating of the tooth which results in patient discomfort as well as pulp damage due to heating. These are all undesirable drawbacks.
Levy proposes using a 1.06 .mu.m wavelength in the form of pulses having an energy content of between 10 and 50 millijoules, with a pulse duration on the order of 100-300 microseconds, and a repetition rate on the order of 50 Hz. Levy states that laser radiation at a wavelength of 1.06 .mu.m which can be produced by an Nd:YAG laser, can be used for cutting or vaporizing demineralized, i.e., decayed enamel and dentin, without endangering gum tissue. However, Levy states that laser radiation at a wavelength of 0.532 .mu.m can also be used, but this requires great care because it has been found that radiation at this wavelength will also cut gum tissue. Therefore, radiation at this wavelength, Levy concludes, can be used when it is desired to cut gum tissue. Levy further states that radiation at a wavelength of 0.532 .mu.m has been found to be effective only if applied at dangerously high energy levels. These statements in Levy highlight some of the previously believed disadvantages of using a 0.532 .mu.m wavelength, which corresponds to the frequency doubled output of an Nd:YAG laser.
Levy further discloses that 1.06 .mu.m can be used to cut healthier, mineralized dentin and healthy enamel if a dark colored region is first provided at the spot where cutting is to begin. Levy further discloses that when the radiation is applied to demineralized enamel or pathological dentin, a dark spot is not necessary.
In effect, Levy appears to suggest merely coating the surface of a tooth with a dark substance. Presumably, Levy is suggesting something similar to what is disclosed in the article entitled "Scanning Electron Microscopic Study of Laser-Induced Orphologic Changes of a Coated Enamel Surface", Hess, John, Lasers in Surgery and Medicine, 10:458-462 (1990), where it is disclosed that the use of a waterproof India ink is suitable for use as a laser initiator with a 1.06 .mu.m wavelength output of an Nd:YAG laser. However, this substance is used to merely coat the tooth and does not selectively attach itself to decayed areas.
While it is clear that several attempts have been made to enable a laser to be used to eradicate tooth decay, various drawbacks still exist. In order to overcome some of the drawbacks, the present invention proposes the use of a selected laser in combination with a substance which selectively attaches itself to tooth decay to concentrate the laser energy on the decayed portions of the tooth. While at least one substance which attaches itself to tooth decay is known, such a substance has been proposed for use merely to visibly detect caries. For example, Acid Red 52 in a solvent, e.g. propylene glycol, has been used to visibly detect the existence of caries. Acid Red 52 in propylene glycol is believed to be advantageous for this purpose because the outer carious dentin is stainable by the solution because the outer carious dentin has collagen fibers which are loosened by the irreversible breakdown of the intermolecular crosslinks, whereas the inner carious dentin and normal dentin, which are unstainable, have solid collagen fibers with undisturbed molecular structure. This is disclosed, e.g. in an article entitled "Clinical Guide For Removing Caries Using a Caries Detecting Solution", Fusayama, T., Quintessence International, Vol. 19, No. 6/1988, p. 397-401.
However, this article appears to simply propose the use of this solution to enable visualization of caries while reducing enamel with a high-speed air turbine. With such a turbine device, however, it is said to be necessary to use water to prevent pain caused by friction heat. It is further disclosed that the carious dentin must be reduced at low speed with a round steel bit since the use of a high-speed turbine is apt to induce pain. These are all obviously undesirable drawbacks. Moreover, it suggests that Fusayama fails to recognize that Acid Red 52 can be used in combination with an appropriately selected laser to remove tooth decay.